(1) Field of the Invention
The present invention relates to battery devices and charge control.
(2) Background
With recent increase in interest in energy conservation, energy harvesting is attracting attention. “Energy harvesting” is a technique of recovering and re-using as electrical power natural energy such as solar, wind, hydro, and geothermal energy, or energy such as vibrations, heat, light, and electromagnetic waves released into the surrounding environment from artificial systems such as machines and factories. Energy harvesting is expected to be of use not only in a power source for sensors, mobile devices, etc., but also in an auxiliary power source in equipment using a commercial power source as a primary power source. For example, the image forming apparatus disclosed in Japanese Patent Application Publication 2013-025280 reuses waste heat from a fixer by using a thermoelectric transducer in order to drive an exhaust fan or raise/maintain the temperature of the fixer.
Typical characteristics of energy harvesting are that power generation per generator element is low, in a μW to mW range, and that it is unstable, dependent on environmental conditions such as sunlight amount, weather, and ambient temperature. Based on these characteristics, power generation technology to which energy harvesting is applied typically uses maximum power point tracking (MPPT) in power control of a power generator, and a rechargeable battery in power supply to a load.
Charging of the rechargeable battery typically continues until the battery reaches the full charge. For examples, see Japanese Patent Application Publications H08-308123 and 2006-165383. This charging is controlled by a combination of constant-current charging and full charge detection by a −ΔV or dT/dt scheme, or constant-current constant-voltage charging. Power stored in the battery is supplied to the load when required.